Known control systems for multiphase rotary electric machines are designed to drive a drive system comprising a plurality of switching elements connected to a multiphase rotary electric machine to thereby control rotation of the multiphase rotary electric machine. For instance, FIG. 1 shows a typical drive system for a multiphase rotary electric machine. The drive system comprises a plurality of switching elements 102a, 102b, 102c and 104a, 104b and 104c for a three phase rotary electric machine. One set of switching elements 102a, 102b, 102c is known as the high side switching elements and the other set of switching elements 104a, 104b, 104c is known as the low side switching elements. The control system is designed to drive a pair of high and low side switching elements for each phase winding of the machine to output a sinusoidal voltage to be applied to each of the three phase windings of the motor. This allows torque to be created in the three phase windings with little ripple. A shunt resistor 108a, 108b, 108c is provided in series with each of the low side switching elements 104a, 104b, 104c to provide a cost effective way of measuring current.
FIG. 2 shows how the three phase currents I_U, I_V, I_W (also referred to herein as Ia, Ib and Ic) alter with the electrical angle. Prior art methods select two raw current measurements and deduce the third based on the fact that all three phase currents must sum to zero. These methods assume that the phase difference between the voltages and currents will be small and thus select the phase with the most positive voltage to be calculated from the other two. Hence the raw data used in the calculation changes three times per electrical cycle. There will be a discontinuity in the calculated currents at the three section boundaries if there are any offset errors present in the raw measurements.
The use of shunts has various disadvantages. Current can only be measured in a particular phase when it flows either through the low side switching element 104n or its anti-parallel diode 106n which necessitates deducing the third current from the other two phases. Any offset errors in the raw current measurements result in discontinuities which greatly affect the performance of the closed loop current controllers. The raw current measurements are subject to large spikes directly after the switching instance due to cable charging currents. The current should be sampled a delay after the lower switching element has begun to conduct to prevent the spikes from affecting the measurement. This delay, added to the time required to perform the analogue conversion, results in a minimum time during which the lower switching element must conduct so that the raw measurement can be used. This issue becomes of more concern when the modulation index is high as the time during which current flows through the shunt resistors at certain points in the electrical cycle becomes short.
Inverters that employ bootstrap supplies for the high switching elements require that the lower switching element is on for enough time to charge the bootstrap supply within the period during which the supply would discharge. The insertion of large “charging lower on periods” reduces the robustness of closed loop current controllers.